Why does Marvel/Disney want to share the MCU with these other studios? What good does it do for Marvel/Disney to share? What is the upside? "Oh boy, we get to include Spiderman in the MCU!" So? The MCU is doing just fine without the properties currently housed at other studios, so from a purely business perspective, why include them?

Marvel/Disney is in the position of power for including these other characters in the MCU, because Marvel/Disney owns the MCU. They will not pay to bring Spiderman or whatever properties back, it will be the other way around. Spiderman will only get bigger with an inclusion in the MCU while the MCU will keep plugging along like nothing changed.